doc: Update VRF documentation metric

Two things:

1) Update examples to show usage of metric
2) Discuss reasoning for using such a high metric.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
Acked-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>